Time domain reflectometry (TDR) is a widely-used measurement technique for determining the characteristics of an electrical line, such as a broadband internet cable. In general, a TDR analysis involves propagating a stimulus signal, typically a step or an impulse signal, into the line being measured, then measuring the signal that is reflected back from any electrical discontinuities in the line. The amplitude of the reflected signal may be used to determine the impedance of the discontinuity and the time it takes for the reflected signal to return may be used to determine the physical location of the discontinuity in the line.
A TDR measurement may be performed using a general-purpose test and measurement instrument, such as a DSA8300 series sampling oscilloscope manufactured by Tektronix, Inc. Such an instrument may be configured with a specialized TDR hardware module, and may run specialized TDR software to guide the user in properly connecting the instrument to the device-under-test (DUT) and to calculate the measurement results. A user of such an instrument typically connects the instrument to the DUT using a probing system designed to perform TDR measurements.
One challenge in designing a TDR probing system is that the inputs of instruments used to perform TDR measurements are generally necessarily highly sensitive in order to accurately measure the typically low amplitude reflected signals. For example, the inputs of a TDR module in a DSA8300 oscilloscope have an input DC voltage limit of about 2-3 Volts. The module can suffer permanent damage if subjected to higher voltages. Due to this high sensitivity, TDR modules are also especially prone to damage from electro-static discharge (ESD). In fact, a common field failure for TDR modules is ESD damage caused when a user connects a TDR probe to a DUT. Any electrical charge that may have built up on the DUT is immediately conducted by the probe to the inputs of the TDR module. Therefore, what is needed is a TDR probe designed to protect the instrument from ESD damage.